Please help me i dont understand how to find out if numbers are prime how do i have to do to find out

Say for example you have the number 139. If it is not prime it's divisible by some number less than . So start making a list:

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139 is not divisible by any of the numbers 2, 3, ..., 12. Thus 139 is prime. (Actually you don't need to divide by all of 2, 3, ..., 12, you may simply divide by all primes less than 12: 2, 3, 5, 7, and 11.)

basically...

bear with me, i'm on the other side of the world so i just hope that our math terminologies are the same.

topsquark's post is very helpful, i didn't kow that first part! basically a prime number is something that has only two factors, being 1 and itself. our lovely mathematicians are always quarreling over whether or not 1 is prime, but when i last checked, one was not prime. just check with your teacher though.

bear with me, i'm on the other side of the world so i just hope that our math terminologies are the same.

topsquark's post is very helpful, i didn't kow that first part! basically a prime number is something that has only two factors, being 1 and itself. our lovely mathematicians are always quarreling over whether or not 1 is prime, but when i last checked, one was not prime. just check with your teacher though.

The definition of a prime natural number is: A natural number is prime if it has exctly
two distinct factors.

Suppose N is not prime then it does not have exactly two distinct factors.
Then it is either 1, which has only one distinct factor:-1, or it has more than
two distinct factors (as it must have at least two, 1 and itself).

Let a be the smallest proper factor of N, then:

N=a.b

for some integer b.

Suppose a>sqrt(N), then as b>=a a.b>sqrt(N).sqrt(N)=N a contradiction.

Therefore the smallest proper factor of a composite number N (composite
number: a non-prime with more than 2 distinct factors) is less than of
equal to sqrt(N).